Bolometers or bolometric detectors detect an increase in temperature caused by IR absorption of an object as a change in resistance. If an object is made of a metal, the resistance increases in response to the temperature increase. If an object is made of a semiconductor, the resistance decreases in response to the temperature increase. As materials used for the bolometers, metals such as titanium (Ti) are sometimes used, but semiconductors such as amorphous silicon (a-Si), vanadium oxide (VOx), and titanium oxide (TiOx) are mainly used, because the semiconductors have a higher temperature coefficient of resistance (TCR) than the metals, and thus are suitable for resistive materials for bolometic detectors. At cryogenic temperatures, superconducting detectors have been employed to achieve background limited sensor performance in the far-to-mid infrared.
Impedance matched coatings or films are used for absorber applications to couple mid-to-far infrared (IR) radiation to an ultra-sensitive bolometric detector suspended on an ultrathin (˜1 micron thick) dielectric membrane, for example Si. The unique aspect of cryogenic bolometric detectors is that in order to provide adequate responsivity and speed, their heat capacity must be low. Furthermore, in order to achieve an optimal signal to noise ratio, spectral filtering of the incident radiation is desirable. Conventional approaches for identifying absorber coatings are generally not acceptable, because (1) they are susceptible to aging (Ti/Au, Bi), which results in a transient optical efficiency of the instrument, (2) they have high heat capacity (foams like gold black, multi-walled carbon nanotubes, or meta-material coatings) which impact the performance of the low-background cryogenic detectors, (3) they are reactive in the short wavelength limit (Bi), which correspondingly reduces their coupling efficiency, (4) or add considerable fabrication complexity (e.g., performing an implant on a silicon (Si) membrane, also referred to as a substrate or detector substrate) for some detector architectures. Uniform thin film coatings which are not superconducting usually do not provide the desired spectral filtering attributes. Patterning the layer to realize such an objective can be employed, however, places the absorber metallization at risk. Furthermore, some thin film absorber coatings have extremely high intrinsic stress, which can cause the dielectric detector membranes or substrates to break or bend in an unwanted fashion causing damage thereto.
Among the desired properties of such absorber coatings or films are low intrinsic stress, which makes them mechanically compatible with integration on ultra-thin dielectric membranes and they can be made to possess the optical impedance required for high optical efficiency absorption.